Electrical insulation



Patented Feh. Z2, 1

UNITED 2,342,522 nmo'rmcsr. INSULATION Kenneth H. Barnard, Andover,Mass, assignor Pacific Mills, Lawrence, Mass, a corporation ofMassachusetts No Di-swing.

Application March 11, Serial No. 281,301. I i

'7 ZClalms. (on. m- -ss) v and low power factor.

Cotton fabric is used extensively tor the insulation of electricalconductors both in the form of sheets and tapes. While cotton issometimes used for insulation purposes without the addition oi aninsulating varnish or compound, for most purposes the sheets areimpregnated orcoeted with some varnish or compound. 1

In the manufacture oi so-cslled cemoric mo e which has as smooth surfacefinish, the fabric is first filled with starch end celendered end thenis coated with o suitable varnish. Such tones are extensively used inthe insulation or high tension power conductors.

While the various insulating varnishes odd to the electrical resistanceoi the tape, the reslst= once of the cloth is o vitol rector. Urdinew insuloting camhrle rosy hove e. resistance of one thousand ohms percentimeter cube tested end measured as explained hereinloelow.

In the manufacture oi cambrio tapes the or dinsry cotton fehrlc issinged to remove the pro jecting surface :Zihr prior to the spplicstlor.of the starch and vs. h otherwise the ends of these fibres would e..varnish to the swrfsce of the tape one the electrical insulation thereofdue to the poor er resists-nee oi the someone.

I have discovered it method of treating cotton fabric day whichinsulation thereof is renter ably inoreesed, Erom 30 to lilil times.over the resistance or the ordlhery fabric as used for insulletion astested in the manner explained hereinsfter.

My camhric is on entirely new srticle oi mime I. techno and issubstantially non-conducting, es-

pecially as compared with osmbric heretofore used as insulation. I

I manufacture my improved camhrlc as follows:

Ordinary cotton gray cloth, either singed or not, as desired, is boiledwith dilute caustic soda in a kier for eight hours at 15 pounds steampressure. The cloth is then washed in ordinary water soured in dilutesulphuric acid and rinsed with water until iree from acid. All thisfollows wellknow practice.

The cloth is next washed repeatedly with distilled water of testedpurity to wash out all impurities soluble in water until samples of thesend through the storch and iii) cloth when tested as hereinafterdescribed show resistance of 50,000 ohms per centimeter cube or more.

A starch finish is necessary for certain. 'pur- Poses as intheproduction of cambric tape. 'In case a. starched finish. is desiredsuitable for costing with an insulating varnish and proper slitting, Ipurity the starch itself in a manner similar to that used for the cloth,for example:

I take 400 pounds of potato starch end wash it repeatedly with ordinarycold water by decantstion to remove the more soluble impurities. This isfollowed by repeated washing with dis tilled water of tested purityuntil the lost water extracts give e test of approximately 50,000 ohmsper centimeter cube or more. The starch is then seedy for use after thesupernstent liquid is de cen'ted oh. and is boiled upwith distilledwater. "using from 25 to 100 pounds of the purified starch per hundredgallons of distilled water. This purifieol starch mix is applied to thepurified cloth on ,e mangle, followed by framing to width end dryins" inthe usual manner. The cloth is finally given s. highly ealendered finishon e. friction eolender in order to get the smooth glossy our loserequired Toy the electrical trade. lot can then be hosted with a,desirable varnish.

in testing the cloth for its electrlcsl cohductivlty, or resistance, To.10 gram sample or the fabric is boiled for 5 minutes in a clean glossflash with c. o. of conductivity water having. 9. resistance of sround300,000 chins per centimeter euse. Thesis to say. "conductivity" wateris wodistilled under conditions to remove electrioolly conductingconstituents soluble in water therefrom and to bring it to a, eleetricslre sletonce. conductivity woter can be increased over sooooo per centercube it can only he done o her very special conditions which ere not neowith the present invention.

The sample oi conductivity water in which the semple of the treatedfabric has been loclled is cooled to 20 0. end is measured for itselectrical resistance by a. Leeds 8; Northrup conductivity meter orsimilar apparatus and for the fabric prepared loy the above method theresistivity of the water will he found to be about 50,000 ohms percentimeter cube. That is to say, the fabric does not impart to the waterany material which seriomiy reduces its electrical resistance.

When ordinary cotton fabric as heretofore used for insulation purposesis tested under the same conditions it is found that the test solutionhas a resistivity of from 500 to 1500 ohms per centi- Whlle theelectrical resistance of the meter cube, showing that my improved fabrichas a much higher electrical resistance.

It might be stated that it is not practical to measure the electricalresistance of the fabric directly between terminals as the measurementdepends upon the area of contact between the fabric and the terminalsand this area of contact is so exceedingly variable that dependableresults cannot be obtained and the above described method of testing hasproven by experience to be the most reliable method.

It is quite probable that my method of improving the electricalinsulation by washing the fabric in distilled highresistance water notonly dissolves out of the fibres the water-solubleelectrically-conducting, or, considered from a nalternating currentstandpoint, electrically-absorptive salts, and the like, that the cottonplant abstracts from the soil in'which it grows and which impairs theelectrical resistance of the fibres, but also does not implantconducting material in the fibres as would be the case if ordinary washwater from a lake or river was used.

Not only is the dielectricbreak down resistance of the fabric and thetape improved but tests show that the power factor of the tape isgreatly improved. Where varnished tape made from ordinary fabric has apower factor of about 4%, my improved varnished tape has a power factorof only about 2%. This is a marked improvement.

In an insulating material the power factor is the measure of theelectrical losses which are represented mainly by heat generated in theinsulating material. The life of an insulated cable is dependent uponthe useful life of thematerial by which the conductor is insulated. Theinsulating material deteriorates mainly by heat where the material isnot exposed to the weather and mechanical wear. The conductor of thecable does not ordinarily become heated to impair the insulation andhence the life of the insulation depends mainly upon the heat generatedinternally in the insulatingmaterial by reason of the power factor loss.Thus by halving the power factor loss by the use of my improvedinsulating material over the ordinary material a much longer useful lifeof the insulated conductor can be expected.

I claim:

1. The method of making an electric insulating .fabric containingstarch, which includes the step of extracting the electricallyabsorptive and conductive materials from the starch by steeping it indistilled high resistance water and combining the resulting starch witha fabric from which sirnilar materials have been extracted.

2. The method of manufacturing an electric insulating materialcomprising a cotton fabric containing starch, which includes the stepsof extracting the electrically absorptive and conductive materials fromthe fabric with distilled water, whereby the electrical resistance ofthe fabric is markedly increased, extracting the electrically absorptiveand conductive materials from the starch with distilled water, wherebythe electrical resistance thereof is markedly increased, and applyingthe treated starch to the treated fabric.

xnmrn'm H. BARNARD.

